Lec. 8
General description of meiosisAfter several mitotic divisions the spermatogonia or oogonia, meiotic division starts. In the G2 period of the interphase there is apparently a decisive that direct the cell toward meiosis in stat of mitosis and this take place at the beginning of G2 as shown in some experiment.
Meiosis require two nuclear divisions {meiosis I ,and meiosis II} and produce four haploid daughter cells ,each having one of each kind of the chromosome and therefore the total number of chromosome present in the diploid parent nucleus ,while the daughter cells have the haploid number of the chromosome .The following are the essential process of meiosis :
The pairing {synapses} of the homologous chromosome.
The formation of chismata which represent there underlining genetic recombination {the interchange of genetic material}.
The segregation of the homologous chromosome.
The first meiotic division is characterized by long prophase, during which homologous chromosomes pair closely and interchange hereditary materials and it is subdivided into five phases:
1-Prophase
Leptonema { lepto :thin,nema:thread} ,in which the nucleus has increased in size and the chromosomes have become more apparent .The Leptonema chromosomes differ in the following two points from the prophase mitotic ones :DNA duplicated has occurred ,and they have two chromatids ,leptoten chromosomes look single rather than double .
These chromosomes show bead-like thickenings {called chromomer}which appear at irregular intervals along their length .Since these beads are characteristic in size ,number ,and position for a particular chromosome , they used as a landmarks to identify a specific chromosome of an organism.
B-Zygonema {zygon: adjoining }: in this stage ,homologous chromosomes {i.e. have the same length and centromer position ,and contain the same types of genes } become aligned and undergo pairing in a process called synapses {synapse :union }of the chromosomes ,pairing involves the formation of a specific structure named as the synaptonemal complex {SC} which appeared {at the end of Leptonema }in the space between the two chromatids .
C-Pachynema {puchusi : thick} the chromosomes contract longitudinally resulting in shorter and thicker thread ,and each chromosome divides longitudinally except at one region {the centrosome } where it remains single and contains four chromatids ,each of homologue are called sister chromatids {tetrad} or bivalent .Each homologous chromosome has an independent centromere ,in late pachytene ,four chromatide become visible ,the chromatid of each homologue are called sister chromatids and the space occupied by the {SC } is maintained .Pachynema is usually along lasting stage of prophase ,while Leptonema and Zygonema may last a few hours ,it may last for days ,weeks ,or even years .
D- Diplonema :homologous chromosomes remain united by their points of interchange ,or chiasmata { i.e. cross piece } .Chiasmata are generally regarded as the expression of phenomenon called crossing over ,or recombination ,by which chromosomal segments with blocks of genes are exchanged between homologous members of the pairs {between chromatids of different and homologous chromosomes } .Some chromosomes have one chiasmata and other have several .During this stage the four chromatids become visible .SC complex can no longer be observed .Diplonema is a long lasting period ,for example , human oocytes reached this stage in the fifth month of prenatal life ,and remain in it until many years later until ovulation occurs .
E-Diakinesis {dia : across} the contractions of chromosomes again become accentuated ,tetrads are more distributed in the nucleus ,nucleolus disappears .During this period the number of chiasmata are diminishes .
2-Metaphase
A-Prometaphase I : Condensation of the chromosomes reaches into maximum ,the nuclear envelop breakdown ,and the spindle microtubules become attached to the centromeres ,each homologue is attached to one of the poles by the homologous centromere and the two sister chromatids behave as a functional unit.B-Metaphase I: The bivalents held together by chiasmata at their have moved toward the metaphase plate {equator of the spindle}.Kinetochores, regions just outside the centromeres are attached to spindle fibers called kinetochore spindle fibers.
3-Anaphase I: The sister chromatids of each homologue, united by their centromere, their composition is different from that of the originals, move toward their respective poles.
4-Telophase I: Begins when the anaphase groups arrive at their respective poles, in some species, there is a Telophase I stage at the end of meiosis I. If so, the nuclear envelope reform and nucleoli appear .This phase may or may not be accompanied by cytokinesis.
5-Interkinesis Is similar to interphase in mitotic division except there is no DNA replication, the chromosomes are already duplicated, these are now haploid in number, although each one consists of two chromatids.
Meiosis II
The short prophase II is followed by the formation of the spindle, which marks the beginning of metaphase II, at this stage, chromosomes become arranged on the equatorial plane, and the sister centromeres separate, the two sister chromatids go toward the opposite poles during anaphase II.Since the longitudinal halves of each parental chromosome separate in this division each of the four nuclei of Telophase II has one chromatid and each nucleus has a haploid number of chromosomes .Each one of the four gametes has a different genetic constitution as a result of the crossing over ,each chromosomes does not consist solely of maternal or parental
Material but of alternating segments of each.
The random assortment of the genes is due not only to crossing over but also to the fact that the distribution of the chromosomes in the first and second division is normally random, in this process, genetic variation is produced.